Traditionally, hydroforming techniques are used to draw and shape metal tubes. Conventional hydroforming techniques often involve two steps: (1) placing the desired bends in the tube and (2) forming the tube to the desired configuration. Step 2 usually requires placing a tubular member having an open bore in a mold and pinching off the ends of the tube. A pressurized liquid is injected into the open bore, causing the tube to stretch and expand out against the mold.
The advantage of the hydroforming process is that it allows formation of relatively long tubular structures having a seamless perimeter. This process eliminates the cost of welding or machining operations often used to shape the part in the desired configuration. As a result, a hydroform oftentimes has a high length to diameter ratio. For instance, a hydroform structure may have a length in excess of 15′ and a diameter ranging from approximately ¾ to more than 12″. It is not unusual for a hydroform structure to exceed the length of other tubular members, such as torsion bars or tubular bars, formed using other processes.
Additionally, hydroforms are complex structural shapes that typically include bends and contour changes. Often the number of bends and contour changes in a hydroformed bar are greater and more complex than those found in torsion bars or other tubular structures formed using different techniques.
Hydroform structures typically have a constant wall thickness prior to forming, and thus tend to develop weak spots at the site of bends or changes in contour, as well as at certain locations along a long tubular section. Thus, hydroform sections are generally reinforced to improve their structural stiffness and strength.
Traditional ways of reinforcing tubular structures such as hydroforms include sliding a metal sleeve inside the tube and welding the reinforcing member in place. However, because the hydroform often includes one or more bends or one or more changes in contour, it is often difficult to insert the sleeve into the hydroform at the site of weak spots. Other techniques include reinforcing the hydroform from the outside by welding the sleeve onto the outside of the hydroform. However, hydroforms are often used in applications having very close tolerances, resulting in little or no clearance for an externally placed reinforcing member.
Additionally, in many operations the weight of the tubular member is critical and must be kept low as possible. Thus, the use of an external sleeve adds unwanted weight to the tubular assembly. Finally, the welding operation tends to be labor intensive, time consuming and inexact, increasing the cost of forming the hydroform member and producing parts that have questionable reliability.
Consequently, there is needed a device and method for reinforcing the weak areas of hydroform tubes without significantly increasing the weight and manufacturing complexity.